Ionically charged webs (e.g., polymeric webs) are common in web handling operations, where the web moves over and around various rollers, bars, and other web handling equipment. Ionic charge (i.e., static) builds up on the web from many causes including the contact and separation of the web from the various rolls and equipment.
Electrostatic charges on a web can produce a number of product quality damaging web coating problems. These charges can be very detrimental in the area of precision coating not only because of spark ignition hazards, but also because these static electricity charges can cause a subsequently coated liquid layer to be disrupted and form undesirable patterns. In addition to inhomogeneous charge patterns, homogeneous charge can also generate coating defects. These charge patterns can cause defects in processes such as coating and drying,
In the photographic industry, for example, a significant non-uniform thickness distribution of a photographic coating material often results when such material is applied to a randomly charged web. Because of the high surface resistivity of high dielectric materials, such as polyester based materials and the like, used in photographic film, it is fairly common to have relatively high polarization and surface charge levels, of varying intensity and polarity, occupying web areas closely adjacent one another. The use of such coating materials as a component of a photographic positive or negative, for example, often requires the use of relatively thick coatings to provide at least a minimum thickness coating throughout the web and thereby compensate for such non-uniform thickness distribution which necessarily results in an increase in the use of relatively costly coating materials in order to produce an effective coating thickness. Visual effects such as mottle are also a consequence of coating non-uniformly charged webs. Past practices included either tolerating this non-uniform charge distribution and its disadvantages or attempting to neutralize a randomly charged web as much as possible prior to applying the coating materials.
Various techniques for supposedly neutralizing charged webs are known.
One old technique, described in U.S. Pat. No. 2,952,559, involves passing a charged web between a pair of opposed grounded pressure rollers that are spring-force biased against opposite web surfaces for the purpose of neutralizing bounded or polarization-type electrostatic charges and then blowing ionized air onto surfaces of the web to first neutralize surface charges and then establish a particular web surface charge level prior to coating same. This resulting surface charge level is compensated for by applying a voltage to the coating applicator during the actual coating process having a polarity that is opposite to that of the web surface charge.
Another technique, described in U.S. Pat. No. 3,730,753, involves “flooding” a web surface with charged particles of a first polarity so as to generally uniformly charge the surface and thereafter removing the charge imparted to said web surface so as to leave the surface generally free of charge. The amount of charge added to and/or the amount of charge removed from the web surface may be so controlled that the charge variation and the net charge on the surface is lowered to an acceptable low level.
However, the detrimental effects on precision coating may occur even when the homogeneous charge is balanced to give a net zero charge. To ensure that static charge on webs does not adversely affect the coating and/or drying process, it is desirable to precisely neutralize webs in continuous processes. This is currently not possible using commercially available neutralization systems.
Commercially available neutralization systems, which are useful but do not solve the problem, include:
Air Ionizers, which provide a source of ionized air. Air naturally contains ions. However, these ions are not sufficiently abundant in most cases to neutralize static charges rapidly enough to protect static sensitive devices. Further, air ions are removed by HEPA and ULPA filters in clean rooms.
Electrical Static Eliminators, which consist of one or more electrodes and a high voltage power supply. Ion generation from electrical static eliminators occurs in the air space surrounding the high voltage electrodes. There are various commercial sources for electrical static eliminators, such as MKS Ion Systems and Simco (an Illinois Tool Works company).
Induction Static Eliminators are passive devices that generate ions in response to the electric field emanating from a charged object. Examples of common induction static eliminators include Static String™, tinsel, needle bars, and carbon brushes.
Nuclear Static Eliminators, which create ions by the irradiation of air molecules. Most models use an alpha particle emitting isotope to create ion pairs to neutralize static charges. These are often also called Nuclear Bars.
Each of these neutralization systems provide a means to attain a web that is net neutralized (i.e. the magnitude of electric field, as measured with a common static meter is substantially lower than was initially, provided the initial charge was substantial). However, the net neutralized web may still have substantial charge.